The present invention relates to a switching power supply.
A self-oscillating switching power supply which utilizes the principle of a blocking oscillator has an advantage in that circuit arrangements are simple. In such a self-oscillating type switching power supply, in order to electrically isolate the primary side from the secondary side, a photocoupler is generally used as described in Japanese Utility Model Examined Publication No. 59-32224 (1984). In other words, in the prior art self-oscillating type switching power supply, as described in this Japanese Utility Model Unexamined Publication, such a system is employed in which an analog signal representative of a detected output of the switching power supply is applied to a photocoupler, and the oscillation of a switching element is controlled by an analog output generated from the photocoupler in accordance with the analog signal. However, in this system, since the signal transmission gain of the photocoupler is small, sufficient precision of the output control of the switching power supply can not be obtained. The precision has been about .+-.5%.
Moreover, there is a drawback that the photocoupler which can be used as a switching power supply must be safe, providing sufficiently high dielectric strength between the primary side and the secondary side, and consequently, a photocoupler satisfying such requirements is expensive. Further, another drawback is that since a photo diode in the photocoupler is required to be supplied with a current equal to 100 mA or more, the power consumption in the control circuit increases, and thus the efficiency becomes low.
In Japanese Utility Model Unexamined Publication No. 56-52489 (1981), there is disclosed a DC-DC converter in which the length of an ON period of a switching transistor is controlled by using a saturable reactor. In such a DC-DC converter, however, there is a drawback in that since it is not the self-coscillating type, a control circuit for the switching transistor is complicated. In the present invention, the switching transistor is controlled also by using the saturable reactor, however, inventors of the present application have established a circuit arrangement using a saturable reactor in the self-oscillating switching power supply, and have solved problems inherent in using the saturable reactor in the self-oscillating type as will be described later.